where the angular integration is performed over the directions satisfying \(|\Delta ( {k} )|\le E\) .
Thus, at low temperatures the susceptibility is entirely determined by the thermally excited quasiparticles near the gap nodes and has the following asymptotics:[1]}, [2]}
\(\chi (T)\propto \left\lbrace  \begin{array}{ll}e^{-\Delta /T}, & \mathrm {fully\ gapped},\\T^{2/k}, & k^{\mathrm {th}}\ \mathrm {order\ point\ nodes},\\T, & \mathrm {line\ nodes}.\end{array} \right.\) 
